There are a number of applications in electronics which require a reference voltage. For example, digital to analog converters (DACs) require accurate reference voltages in order to properly convert digital signals to analog signals. A voltage reference circuit widely used in integrated circuits is known as a “bandgap reference circuit” and has an output voltage of around 1.25 volts with little temperature dependence. Bandgap reference circuits maintain two internal voltage sources, one of which has a positive temperature coefficient and another of which has a negative temperature coefficient. By summing the two voltage sources together the temperature dependence of the output of the bandgap reference circuit is reduced.
A common type of bandgap reference circuit known as a “Brokaw” bandgap reference circuit uses negative feedback (implemented, for example, by an operational amplifier) to force a constant current through two matched bipolar transistors with different emitter areas. The transistor with the larger emitter area requires a smaller base-voltage with the same current and the base/emitter voltage for either transistor has a negative coefficient (i.e. it decreases with temperature). Also, the difference between the two base-emitter voltages has a positive temperature coefficient (i.e. increases with temperature). The circuit produces a “proportional to absolute temperature” (PTAT) current through the series connection of a first resistor and a second resistor. When the ratio between the first and second resistors is chosen properly, the circuit's first order effects of temperature dependency will be canceled out due to the complimentary to absolute temperature (CTAT) characteristics of the base-emitter voltages of the transistors.
In the prior art, the bandgap voltage reference is “trimmed” such that the apex of the inverted parabola is at approximately room temperature (25° C.). As the operating temperature of the circuit varies from the nominal 25° C. to which the circuit is trimmed, the reference voltage produced by the circuit tends to drop. The voltage output temperature dependency of a Brokaw bandgap reference circuit typically takes the form of an inverted parabola as can be seen by the upper curve of FIG. 3.
For many electronic circuits, the temperature dependence of bandgap reference circuits such as the Brokaw bandgap reference circuit is adequate for the job. However, some electronic devices require a reference voltage which is very stable with respect to temperature. For example, some devices require a voltage reference that does not vary more than about one part per million per degree Celsius (1 PPM° C.).
The aforementioned DACs are particularly sensitive to temperature-dependent reference voltage error. This is because DACs, and especially long word (many bits) DACs require many reference voltage levels to perform their function. This tends to multiply errors because the required multiple reference voltages are typically generated by cascading bandgap reference circuits and/or by amplifying the output of a bandgap reference circuit.
Various techniques have been used to improve the temperature performance of bandgap reference circuits including piecewise linear correction of the bandgap voltage and the use or polyresistors (which have a large second order temperature coefficient) to compensate for temperature variations. However, neither of these techniques is very accurate or repeatable across manufacturing process variations, and they tend to be expensive and/or difficult to implement.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.